Conventionally, apparatuses which obtain radiographic images of objects by irradiating the objects with X-rays and detecting the intensity distributions of X-rays transmitted through the objects have been widely and generally used in the fields of industrial nondestructive testing and medical diagnosis. As a general method for such imaging, a film/screen method using X-rays is available. This method images by using a combination of a photosensitive film and a phosphor having sensitivity to X-rays. Rare-earth phosphor sheets which emit light upon application of X-rays are held in tight contact with the two surfaces of a photosensitive film. The phosphor converts X-rays transmitted through an object into visible light. The photosensitive film then captures the light. The method then develops, by chemical treatment, the latent image formed on the film, thereby visualizing the image.
The recent advances in digital technology have popularized the scheme of obtaining high-quality radiographic images by converting the intensity distribution of X-rays transmitted through an object into an electrical signal, detecting it, processing it, and then reproducing the resultant information as a visible image on a monitor or the like. As such a method of converting a radiographic image into an electrical signal, there has been proposed a radiographic image recording/reproduction system which temporarily stores X-rays transmitted through an object as a latent image in a phosphor, photoelectrically reads out the latent image by irradiating the phosphor with excitation light such as a laser beam, and then outputs the readout image as a visible image.
In addition, with the recent advances in semiconductor process technology, there has been developed an apparatus for capturing a radiographic image in the same manner as described above by using a semiconductor sensor. These systems have very wide dynamic ranges as compared with conventional radiographic systems using photosensitive films, and can obtain radiographic images which are robust against the influences of variations in the X-ray dose upon exposure. At the same time, unlike the conventional photosensitive film scheme, this method need not perform any chemical treatment and can instantly obtain an output image.
Recently, a portable imaging apparatus like that disclosed in Japanese Patent Laid-Open No. 2006-212175 has been proposed. This reference discloses a structure configured to engage pins provided on the housing side in holes provided in the front cover in order to achieve reductions in thickness and weight. In order to achieve weight reduction, Japanese Patent Laid-Open No. 2009-42350 has proposed an apparatus configured to join two exterior members with an electrically disbonding adhesive.
In general, improving the strength of an apparatus itself while protecting the interior of the apparatus will increase the weight and size of the apparatus itself. As shown in FIG. 1, a portable X-ray imaging apparatus is sometimes inserted under an object 6 to image it on a table in an X-ray room, on a bed in a hospital ward, or on an operating table in an operating room. For this reason, an X-ray imaging apparatus 1 is required to be thinner to reduce the discomfort experienced by a patient. When inserting the X-ray imaging apparatus 1 under the object 6 at the time of imaging, an X-ray technician who operates the X-ray imaging apparatus 1 needs to hold the apparatus with one hand. When using an X-ray imaging apparatus on a nursing cart on a side of a bed in a hospital ward, a technician needs to set the X-ray imaging apparatus while holding the posture of a patient by himself/herself. The apparatus is therefore required to be smaller and lighter for a reduction in load on the part of the technician as well.
In the X-ray imaging apparatus 1, the housing for protecting an X-ray sensor 2 accounts for most of the weight. The housing is roughly divided into two parts. In many cases, one part is placed on the X-ray incident side of the X-ray sensor 2, and the other part is placed on the opposite side to the X-ray incidence side of the X-ray sensor 2. A metal material such as an aluminum alloy or a magnesium alloy is often used for the housing on the X-ray incident side to achieve a reduction in weight. CFRP is often used for part in the X-ray incident range because of the necessity to have a high X-ray transmittance. With the recent downsizing of imaging apparatuses, an X-ray transmission member occupies most of the housing on the X-ray incident side. A metal material such as an aluminum alloy or a magnesium alloy is often used for the housing on the opposite side to the X-ray incident side to reduce the influence of external noise on the housed X-ray sensor 2 and reduce the weight.
When different kinds of materials are used for the housing in this manner, since they have different thermal expansion coefficients, a change in the temperature of the housing due to a change in ambient temperature will deform the housing of the imaging unit. When such an apparatus is transported by air or sea, changes in temperature are greater than those in normal usage. This increases the risk of deforming the housing of the imaging unit and imposing adverse effects on the housed X-ray sensor 2. Although it is possible to solve this problem by making a design for increasing the rigidity and suppressing deformation, for example, increasing the thickness of the housing, this contradicts the attempt to reduce the size and weight of the apparatus.
In the apparatus disclosed in Japanese Patent Laid-Open No. 2006-212175, holes are formed in the front cover on the X-ray incident side, and engagement pins are provided on the housing containing the X-ray sensor 2. The pins are fitted in the holes of the front cover to form the exterior of the housing. To prevent the removal of the front cover, the cover is sandwiched between sandwiching members. According to this structure, however, severe machining accuracy is required in terms of a plurality of pins existing along the outer periphery of the housing and the positions of holes of the front cover which engage with the pins, resulting in high component costs. In addition, the following problem is still unsolved. Since different materials are used for the front cover and the housing, and they engage with each other, great changes in ambient temperature at the time of, for example, the transportation of the apparatus will deform the apparatus due to the difference in thermal expansion coefficient between the front cover and the housing. This affects the X-ray sensor 2 inside the apparatus.
In the apparatus disclosed in Japanese Patent Laid-Open No. 2009-42350, although the X-ray sensor 2 is covered by the two exterior members, they are joined to each other with an electrically disbonding adhesive. In some cases, different materials are used for these two exterior members in consideration of X-ray transmission and strength securement. For this reason, as in the case of Japanese Patent Laid-Open No. 2006-212175, great changes in ambient temperature at the time of, for example, the transportation of the apparatus may deform the apparatus due to the difference in thermal expansion coefficient between the front cover and the housing. This may affect the X-ray sensor inside the apparatus. Furthermore, since the exterior members are joined to each other with a special adhesive, it is necessary to use dedicated equipment for separating the two exterior members when performing maintenance inside the apparatus. In addition, bonding the exterior members again after the maintenance will require a cumbersome process of removing the adhesive adhering to the members.